The present invention relates generally to the production of bent sheets of refractory material such as glass, and more particularly to a novel adjustable mold for bending such sheets.
Curved or bent sheets of glass are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures determined by the configuration and size of the vehicle openings as well as the overall styling of the vehicle. At the same time, it is important that the bent sheets meet stringent optical requirements and that the viewing area of the closure or window be free of optical defects that would tend to interfere with clear viewing therethrough. Thus, it can be appreciated that not only is it required to have bending apparatus that will shape glass sheets to precise curvatures, but also that it will do so without causing serious optical defects to the surfaces thereof. Further, bent glass sheets intended for use as glazing closures in vehicles are subjected to an additional heat treatment process to either anneal or temper the glass for strengthening the same and increasing the resistance thereof to damage resulting from impact.
Generally, the commercial production of bent sheets of glass of the above character includes heating pre-trimmed flat sheets of glass to the softening temperature thereof, pressure bending the heated sheets to a desired curvature between male and female mold members having complementary shaping surfaces and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of glass. Such bending of glass has been referred to as "press bending" and may be carried out with the glass sheets supported in either a horizontal, vertical or oblique orientation.
In order to obtain the maximum accuracy of curvature, while maintaining the area of contact between the mold and the glass as low as possible, it has become a general practice to construct the male mold member with a substantially solid or continuous shaping surface which contacts the entire surface of the sheet, and the female mold member with an outline or ring-type shaping surface which contacts only the marginal edge portions of the sheet. Traditionally, mold members have been fabricated of wood, metal or refractory and have been used with various degrees of success.
However, since the introduction of press bending the use of the process has greatly expanded, both in volume and in the number of different shapes produced, and has arrived at a point where it has become evident that the traditional mold per se and the materials to fabricate the same are becoming impractical for a number of reasons. For one, solid mold members are becoming increasingly more expensive to build due in part to the escalating costs of labor and materials. This has become more evident of late since there is a greater variety of shapes to be bent, requiring a greater variety of molds. The expense involved also means that such molds do not lend themselves well to short production runs of specialized shapes, or to experimental and development work, in which a variety of shapes may be tried before one is finally settled on for production purposes.
Another factor is useful life. While wooden molds exhibit good stability characteristics under fluctuating temperature conditions, they tend to burn and break down with extended use. Although aluminum or other metal molds do not burn, they have high coefficients of thermal expansion and tend to distort under the fluctuating temperature conditions commonly encountered in the press bending process. As a result, they must be frequently reworked to maintain the accuracy of the shaping surface, and it is apparent that there is a practical limit to the number of times a mold can be reworked before it is no longer feasible to use it.
The distortion problems associated with the metal molds have been appreciably alleviated with the development of the adjustable surface solid metal mold. Generally, the shaping surface consists of a relatively flexible metal plate that is attached to a rigid backing plate by a series of strategically placed adjustment screws. Slight alterations can be made to the contour of the shaping surface by manipulation of the screws thus permitting corrective measures to be taken to remedy minor distortions to the surface. Likewise, such apparatus may be utilized to produce different configurations that are not too dissimilar from one another, therefore reducing tooling inventory to some extent. However, in the automotive industry, the glazing closures or windows are generally formed of compound bends, i.e., sheets having radii of curvature about mutually perpendicular axes of bending, and the degree of compound curvature that can be applied to a metal plate is very restricted due to the relative inelasticity of metal. If curved beyond its limitations, creases or ripples will form in the surface thereof creating an undesirable shaping surface, thus restricting each apparatus to a limited series of shapes. Also, since only slight alterations may be made to the surface of the flexible plate, they have to be preshaped to the approximate curvature desired during manufacture.
One of the more prominent materials used in the fabrication of solid molds has been cast refractories such as vitrious silica, and these cast refractory molds have admirably served the purposes for which they were designed. Largely due to durability and ease of forming, they are one of the most reasonable for extended runs. However, as previously mentioned, there is often the need for short production runs of specialized shapes requiring a separate mold for each shape. The inventory of molds can become quite extensive and although cast refractory molds are among the most economical to build the expenses involved in their fabrication can become prohibitive, particularly when only a small number of glass sheets of any particular bend or shape are required. Also, in experimental and development work it is often necessary to make numerous and sometimes extensive alterations to a mold before producing an acceptable product, and as previously mentioned there is a practical limit to how often this can be done before it is necessary to replace the mold with another. Obviously, constructing a new mold is expensive and time consuming, adding to the overall cost of ultimately producing a salable product.